This invention relates to torque sensors, for example for use in robot joints.
Robots that are required to manipulate objects, which may for example be industrial or surgical robots, frequently have an arm composed of rigid elements which are linked together in series by a number of flexible joints. The joints could be of any type but are typically revolute joints, or a combination of revolute and prismatic joints. The arm extends from a base, whose location might be fixed or moveable, and terminates in a tool or an attachment for a tool. The tool could, for example be a gripping, cutting, illuminating, irradiating or imaging tool. The final joint in the arm may be termed the wrist. The wrist may permit motion about only a single axis, or it may be a complex or compound articulation, which permits rotation about multiple axes. As disclosed in our co-pending patent application PCT/GB2014/053523, the wrist may provide two roll joints whose axes are generally longitudinal to the arm, separated by two pitch/yaw joints, whose axes are generally transverse to the arm.
In the case of a surgical robot there are a number of important criteria that influence the design of the distal joint(s) of the arm.
1. It is desirable for the arm, and particularly its distal portion where the wrist is located, to be small in size. That allows multiple such robot arms to work in close proximity and hence opens up a wider range of surgical procedures that the arm can perform.
2. It is desirable for the outer profile of the distal portion of the arm to be circularly symmetrical about the length of the arm. This allows the distal portion to be rotated longitudinally without having to be repositioned if it is close to another robot, to some other equipment or to the patient.
3. It is desirably for the joints to be capable of delivering a high torque, so that they can carry heavier tools and deliver high acceleration to the tool tip.
4. It is desirable for the joints to be stiff, with little or no backlash or elasticity, so that when a tool tip has been positioned it will be fixed in position. A conventional approach to minimising backlash is to designate one or more gear elements as sacrificial, but this requires a high level of maintenance, and can result in worn gear particles being liberated within the arm.
5. It is desirable for all articulations to have position and force/torque sensors, so that the control mechanism can take data from those sensors.
6. It is desirable for the distal portion of the robot arm to be as light as possible, to reduce the force that must be exerted by more proximal joints of the robot arm.
7. A typical robot arm carries cables that provide power to its drive motors and perhaps to a tool, and carry signals back from sensors such as position, torque and imaging sensors. It is desirable for the arm to include a path for such cables to pass in the interior of the arm.
The number of important criteria makes it difficult to design an arm that best balances all the requirements.
One particular problem is how to fit the torque sensors into the wrist of a robot arm. The arrangement should be compact but also allow for high accuracy in torque measurement.
There is a need for an improved torque sensor arrangement for a joint of a robot arm.